1. Field of the Invention
The invention generally relates to oil activated fuel injectors and, more particularly, to a system and method to control spool stroke in oil activated electronically or mechanically controlled fuel injectors.
2. Background Description
There are many types of fuel injectors designed to inject fuel into a combustion chamber of an engine. For example, fuel injectors may be mechanically, electrically or hydraulically controlled in order to inject fuel into the combustion chamber of the engine. In the hydraulically actuated systems, a control valve body may be provided with two, three or four way valve systems, each having grooves or orifices which allow fluid communication between working ports, high pressure ports and venting ports of the control valve body of the fuel injector and the inlet area. The working fluid is typically engine oil or other types of suitable hydraulic fluid which is capable of providing a pressure within the fuel injector in order to begin the process of injecting fuel into the combustion chamber.
In current designs, a driver will deliver a current or voltage to an open side of an open coil solenoid. The magnetic force generated in the open coil solenoid will shift a spool into the open position so as to align grooves or orifices (hereinafter referred to as “grooves”) of the control valve body and the spool. The alignment of the grooves permits the working fluid to flow into an intensifier chamber from an inlet portion of the control valve body (via working ports). The high pressure working fluid then acts on an intensifier piston to compress an intensifier spring and hence compress fuel located within a high pressure plunger chamber. As the pressure in the high pressure plunger chamber increases, the fuel pressure will begin to rise above a needle check valve opening pressure. At the prescribed fuel pressure level, the needle check valve will shift against the needle spring and open the injection holes in a nozzle tip. The fuel will then be injected into the combustion chamber of the engine.
However, in such a conventional system, the spool has a tendency to bounce or repeatedly impact against the open coil during the opening stroke. During this bouncing, it is difficult to control the spool motion and hence results in the inability to efficiently control the supply of fuel to the combustion chamber of the engine. For example, in conventional systems it is not possible to quickly move the spool away from the open coil in order to minimize the bouncing effect during an injection of a pilot quantity of fuel. Accordingly, the initial quantity of fuel provided during the pre-stroke event cannot be easily controllable, resulting in a larger injection quantity of fuel than desired.
This may result in a retarded start of injection, as well as the inability to control the spool and hence the injection of a small, pilot quantity of fuel. That is, during this bouncing or repeated impact, a small quantity (pilot injection) of fuel cannot be metered accurately in order to efficiently inject this pilot quantity of fuel into the combustion chamber of an engine. Additionally, it is also very difficult, if not impossible, to vary the amount of fuel during this pilot injection.
It is also known that the bouncing phenomenon may differ from injector to injector, and over time. For example, different manufacturing tolerances may affect the bouncing phenomenon from, for example, small variations in spool diameter to different coil characteristics. Additionally, over time, in the same injector, variations may result from different operating conditions such as temperature and wear on the parts due to aging and other factors. Thus, the control of fuel quantity may vary from fuel injector to fuel injector, as well as over time with the same fuel injector. This also may lead to higher emissions and engine noise.
In some systems, to provide a smaller quantity of fuel, a delay of the pre-stroke of the plunger is provided. But, in conventional systems this is provided by adding more working fluid, under high pressure, into the injector. The additional pressurized working fluid may cause the appropriate delay; however, additional energy from the high pressure oil pump must be expanded in order to provide this additional working fluid. This leads to an inefficiency in the operations of the fuel injector, itself, and also does not provide a consistent supply of fuel into the engine. Also, this delay does not compensate for variations in fuel injector characteristics over time or from fuel injector to fuel injector, nor does this take into consideration the bouncing effect phenomenon. Thus, this delay may not be an accurate, controllable method for providing small quantities of fuel into the combustion chamber of an engine.
The invention is directed to overcoming one or more of the problems as set forth above.